1. Field of the Invention
The present invention relates to a sliding-mode control scheme, and more particularly to a method, article of manufacture, and system for configuring a controller in the sliding-mode control scheme, including a technique to automatically determine whether or not a sliding-mode control should be applied from least minimum measured experimental data.
2. Description of Related Art
Conventionally, it is often the case that a controller is required to be sufficiently robust against non-linear characteristics and parameter variations/load variations included in a system. As one solution to cope with such requirement, a sliding-mode control (SMC) has been widely used. The SMC causes the state of a controlled object to reach a preset hyperplane and to be constrained thereto while switching the control input, and forces the state to slide to an equilibrium point thereby stabilizing the controlled object against uncertainty, nonlinearity, noises of parameters.
For example, in a position/attitude control of a satellite, a satellite has 6 degrees of freedom of movement of translations and rotations in a 3-dimensional space. Because of the many degrees of freedom, the control needs to be able to cope with variation uncertainties of a nonlinear equation for matching the attitude of the satellite to the attitude of an observation target and for controlling the position of the satellite so that it circles around an observation target while maintaining a constant distance therebetween.
Furthermore, along with the progress of exhaust emission regulation of engines, in order to accurately control an exhaust gas recirculation (EGR) mechanism and the like, a design by a plant model based on a feedback control of multi-input and multi-output system becomes necessary in place of a design based on open-loop control, so that a variable structure system control by a piecewise switching of control input is required.
However, it is often the case that the decision of the introduction of SMC for the targeted control relies on the model designer, and it is problematic that in order for such decision, the existence of nonlinearity needs to be verified based on a great deal of experimental results. Further, in SMC, while the nonlinearity is utilized in the control law for reaching a hyperplane, in a design which assumes an ideal, instantaneous switching of control-input switching, chattering will have occurred in the control input. To avoid that, although various smoothing has been used, it is also problematic that nonlinearity that defines space is not used.
While physical law description often results in nonlinearity, if, based on that, it is assumed that control strategy is also nonlinear to advance the design of the controller, extra cost will be spent on that. Moreover, the need for collecting a large number of experimental data for verifying the necessity of SMC has been a factor of increasing the cost. Further, it is problematic that in a reaching control law in which nonlinearity is not exploited, a sufficient control effect may not be obtained.
Japanese Patent Laid-Open No. 05-80805 discloses a technique for enabling the introduction of sliding-mode control and adaptive control by utilizing a conventional linear control technology, in which the phase surface of a sliding mode is represented by a form in which a torque command determined by conventional linear control is divided by an integration gain K2, and a Liapunov function is adapted to take into consideration estimated values of an inertia, a coefficient of dynamic friction and a gravity term of an object to be controlled, so that respective linear control gains Kp, K1 and K2 are determined so as to always make the Liapunov function negative and an auxiliary input is determined by changing the above described respective estimated values.
Japanese Patent Laid-Open No. 2003-15703 discloses that to provide a control apparatus of a plant capable of identifying a model parameter by modeling a plant to be controlled, and further stabilizing control at the time of executing sliding-mode control by using the identified model parameter, a model parameter identifier calculates a model parameter vector θ in a format in which an updated vector dθ is added to a reference vector θbase of the model parameter, corrects the updated vector dθ by multiplying the past value of at least one element of the updated vector dθ by a predetermined value which is larger than 0 and smaller than 1, and calculates the model parameter vector θ by adding the updated vector dθ after correction to the reference vector θbase.
Further, in the section 3.1 of the first edition of Kenzo Nonami and Hong-Qi Tian, “Sliding-mode Control (in Japanese)”, CORONA PUBLISHING CO LTD., Oct. 20, 1994, a design method of switching hyperplane is described, and in the section 3.2, a design method of sliding-mode controller is described.
However, although the above described references disclose a design technique for sliding-mode control, they do not suggest a technique for easily determining whether or not the sliding-mode control should be applied.